ND filter and light quantity diaphragming device including the same

ABSTRACT

A thin film type ND filter being inexpensive and excelling in durability. There is provided ND filter ( 0 ) comprising transparent substrate ( 1 ) and, superimposed thereon, light absorption films ( 3,5 ) and dielectric films ( 2,4,6 ), wherein the light absorption films  3.5  consist of a composition comprising 1 to 30 wt. % of a metal in pure form and 50 wt. % or more of a saturated oxide of the metal with the balance of compounds of the metal containing lower oxides of the metal. Metal raw material of the light absorption films ( 3,5 ) is selected from among Ti, Cr, Ni, NiCr, NiFe and NiTi. As for the dielectric films ( 2,4,6 ), SiO 2  or Al 2 O 3  is used. Function of reflection prevention is imparted by superimposing the light absorption films ( 3,5 ) and dielectric films ( 2,4,6 ) in predetermined film thicknesses and in predetermined sequence. Alternatively, a reflection prevention layer may be formed on the back of the substrate ( 1 ).

TECHNICAL FIELD

The present invention relates to an ND filter. The ND (neutral density)filter is used for a light amount diaphragm in order to attenuate atransmitted light volume uniformly throughout a visible area.

RELATED ART

In image pickup systems such as cameras or video cameras, when subjectluminance is too high, an amount of light equal to or higher than apredetermined amount could heretofore been fallen on a photo surfaceeven if the aperture is stopped down to the minimum (even if theaperture is brought to the minimum aperture). Therefore, an ND filter isoften attached to a part of an image pickup system to regulate theamount of light incident on the photo surface. In this case, as the NDfilter has spectral characteristics merely to decrease the amount ofincident light, it needs to have uniform transmittance throughout avisible area. In the image pickup systems such as cameras or videocameras, the ND filter based on a plastic film has heretofore been usedfor the purpose of attenuating the light amount uniformly throughout thevisible area.

Recently, thin-film superimposed type ND filters with superior opticalcharacteristics and durability have come into usage, which are describedin Patent document 1 to Patent document 3.

Patent document 1: Japanese Patent Publication Laid-open No. 52-113236

Patent document 2: Japanese Patent Publication Laid-open No. 07-063915

Patent document 3: Japanese Patent Publication Laid-open No. 2003-043211

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Patent document 1 proposes an ND filter comprising alternating layers ofmetal thin films (such as Ti, Ni) and dielectric films (MgF₂). That is,Patent document 1 utilizes metal films as light absorption films.Therefore, the extinction coefficient of the light absorption films ishigh and the thickness of the metal films to produce the ND filter isvery small, so that it is difficult to control the film thickness.Moreover, if the thickness of the light absorption film is small, it isdifficult to obtain reflection prevention effects in light of the designof the optical multilayer films.

Patent document 2 proposes an ND filter comprising alternating layersthat includes two or more kinds of Ti metal oxide films (extinctioncoefficient k: 1.0 to 3.0) and dielectric films (Al₂O₃, SiO₂, MgF₂). InPatent document 2, a lower oxide of Ti (such as TiO, Ti₂O₃, Ti3O₅,Ti₄O₇) is used as a starting material for the light absorption filmcomprising the two or more kinds of Ti metal oxide films. However, whenthis raw material itself is unstable and the light absorption filmcontains many unstable substances such as the lower oxides, the opticalcharacteristics change over time. Moreover, the film needs to be formedat a high temperature of 150° C. or more in order to obtain anextinction coefficient k ranging from 1.0 to 3.0, but there is a problemthat a substrate is heavily damaged if the plastic film is used for thesubstrate. Another problem is that the raw materials of the lower oxidesthemselves are expensive.

Patent document 3 discloses a thin-film ND filter in which lightabsorption films and dielectric films are superimposed on a transparentsubstrate. The light absorption film is formed by deposition with ametal material as a raw material, and a mixed gas including oxygen isintroduced during the film formation, and it contains oxides of themetal material produced while a constant degree of vacuum is maintained.However, the composition of oxides of the metal material contained inthe light absorption film is not clear.

Means for Solving the Problem

In view of the problems of prior art described above, an object of thepresent invention is to provide a thin-film ND filter which isinexpensive and superior in durability. The following measures have beentaken to attain the foregoing object. An ND filter in which lightabsorption films and dielectric films are superimposed on a transparentsubstrate is characterized by a composition of the light absorptionfilms which includes 1 to 30 wt % of a single component of a metal and50 wt % or higher of a saturated oxide component of the metal, and otherresidual components comprising compounds of the metal including loweroxides of the metal.

Preferably, a metal raw material of the light absorption films isselected from Ti, Cr, Ni, NiCr, NiFe and NiTi. Moreover, SiO₂ or Al₂O₃is used for the dielectric films. Preferably, the light absorption filmsand the dielectric films are superimposed with predetermined thicknessand in a predetermined order to provide an reflection preventionfunction. Alternatively, an reflection prevention layer may be providedon one face of the transparent substrate which is opposite to the otherface of the transparent substrate where the light absorption films andthe dielectric films are superimposed. In such a case, the reflectionprevention layer may be formed of a single layer of a light absorptionfilm or a dielectric film. Otherwise, the reflection prevention layermay be formed of a plurality of layers of a light absorption film and adielectric film. Otherwise, the reflection prevention layer may beformed of one or more of layers of thermosetting resin or opticallysetting resin, which is transparent in a visible light range. Such an NDfilter is used in a light quantity diaphragming device.

Advantage of the Invention

According to the present invention, a light absorption film whichcontains a saturated oxide and a pure metal substance as the maincomponents is produced, and the ND filter is produced by a laminatestructure of the light absorption films and the dielectric films. Inother words, the single component of the metal and its saturated oxidecomponent are mainly contained to keep the residual components includinglower oxides of the metal as low as possible, thereby obtaining the NDfilter which is stable in characteristics and over time. For example, ametal film is used as a starting material, and, for example, a properamount of reactive gas (such as O₂, O₂+N₂, O₂+Ar) is added at asubstrate temperature of 100° C., whereby a saturated oxide of the metalcan be introduced in a film formation process. By properly setting filmformation conditions, the proportion of the residual componentsincluding the lower oxides of the metal can be reduced. As the presentND filter contains a high proportion of saturated oxide components inaddition to the single component of the metal, the thickness of thelight absorption film can be greater than that of the light absorptionfilm of the metal simple substance. This makes it easy to design theoptical films of the ND filter and also to control a manufacturingprocess, further allowing an improvement in reliability.

According to the invention described above, the thickness of the lightabsorption film including saturated oxides is greater than that of theND filter which only comprises metal films, such that the film thicknessis easily controlled and the optical characteristics can be highlyreproducible. Further, because there are less unstable components suchas lower oxides in the light absorption film, the reliability of the NDfilter is increased and the film formation conditions can be regulatedeven at a low temperature, so that the optimum light absorption film canbe formed to obtain ND characteristics. Moreover, since the startingmaterials are inexpensive metals, the ND filter can be produced at lowcost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a layer configuration of anND filter according to the present invention.

FIG. 2 is a schematic block diagram showing a vacuum deposition deviceused to produce the ND filter according to the present invention.

FIG. 3 is a table diagram representing film formation conditions for theND filter according to the present invention.

FIG. 4 is an XPS spectral graph showing the composition of a lightabsorption film included in the ND filter according to the presentinvention.

FIG. 5 is a table diagram representing the composition of the lightabsorption film included in the ND filter according to the presentinvention.

FIG. 6 is a table diagram representing the elemental composition of thelight absorption film included in the ND filter according to the presentinvention.

FIG. 7 is a graph showing the optical characteristics of the ND filteraccording to the present invention.

FIG. 8 is a schematic diagram in which the ND filter according to thepresent invention is applied to a light quantity diaphragming device fora camera.

FIG. 9 is a schematic sectional view showing a layer configuration of anND filter according to another embodiment of the present invention.

FIG. 10 is a schematic sectional view showing a layer configuration ofan ND filter according to a further embodiment of the present invention.

FIG. 11 is a schematic perspective exploded diagram of another examplein which the ND filter according to the present invention is applied toa light quantity diaphragming device for a camera.

DESCRIPTION OF REFERENCE NUMBERS

0 ND filter

1 transparent substrate

2 dielectric film

3 light absorption film

4 dielectric film

5 light absorption film

6 dielectric film

7 reflection prevention layer

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will hereinafter be described indetail in reference to the drawings. FIG. 1 is a schematic sectionalview showing a configuration of a thin-film ND filter according to thepresent invention. As shown, the present ND filter 0 is a thin-film typein which light absorption films 3, 5 and dielectric films 2, 4, 6 aresuperimposed on a transparent substrate 1. A characteristic point isthat a composition of the light absorption films 3, 5 includes 1 to 30wt % of a pure component of a metal and 50 wt % or higher of a saturatedoxide component of the metal, and balance components comprisingcompounds of the metal including lower oxides of the metal. Such lightabsorption films 3, 5 can be formed by reactive physical vapordeposition (PVD) including metal material as raw material. The metal rawmaterial of the light absorption films 3, 5 can be selected from Ti, Cr,Ni as well as alloys such as NiCr, NiFe and NiTi. On the other hand,SiO₂ or Al₂O₃ can be used for the dielectric films 2, 4, 6. The lightabsorption films 3, 5 and the dielectric films 2, 4, 6 can be laminatedwith predetermined thicknesses and in a predetermined sequence toprovide the ND filter with a reflection prevention function. Thethin-film ND filter having such a configuration is used in a lightquantity diaphragming device.

Referring continuously to FIG. 1, a specific film configuration of theND filter 0 will be described. First, the transparent substrate 1comprises PET (polyethylene terephthalate) having a thickness of 0.1 mm.However, the present invention is not limited thereto, and polyesterfilms or polycarbonate films other than PET can be used. The polyesterfilm or polycarbonate film such as PET is preferable to a light amountdiaphragm, but glass or plastics transparent in a used wavelength regioncan properly be used as the transparent substrate 1 as long as the usageis not specifically limited. The first dielectric film 2 formed on thetransparent substrate 1 comprises SiO₂, and its physical film thicknessis 59 nm. The first light absorption film 3 formed thereon includesmetal Ti and its saturated oxide TiO₂ as the main components, andcontains, as other residual components, lower oxides such as Ti₂O₃, TiOand by-products such as metal compound TiN. The physical thickness ofthe first light absorption film 3 is 28 nm. The second dielectric film 4formed thereon comprises SiO₂, and its physical film thickness is 51 nm.The second light absorption film 5 formed thereon also includes metal Tiand its saturated oxide TiO₂ as the main components, and contains, asother residual components, lower oxides such as Ti₂O₃, TiO and metalnitride TiN. The physical thickness of the second light absorption film5 is 25 nm. The third dielectric film 6 formed thereon comprises SiO₂,and its physical film thickness is 78 nm. It is to be noted that such alaminate structure is illustrative and does not limit the scope of thepresent invention. In the case of the optical thin film, a transparentceramic material is denoted as the dielectric film in an ordinary usedwavelength. By laminating the dielectric films having the thickness(about several times as large as the wavelength) at which interferenceeffects of light emerges, optical characteristics (such as a reflectionamount, transmitting amount, polarization, phase) of incident light canbe freely adjusted. In the present embodiment, a layer configurationshown in FIG. 1 provides the ND filter with the reflection preventionfunction. On the other hand, the light absorption films literally serveto absorb incident light in the used wavelength region, and a metal isusually used in a visible area. In the present invention, its saturatedoxide is particularly introduced into the metal to improve opticalcharacteristics and physical characteristics.

The ND filter shown in FIG. 1 can be formed, for example, by vacuumdeposition. FIG. 2 is a schematic block diagram showing one example of avacuum deposition device used to produce the ND filter shown in FIG. 1.As shown, the present device is configured mainly with a vacuum chamber11, onto which a film thickness monitor 12 and a film thicknesscontroller 13 are attached. A substrate holder 14 which supports andfixes a substrate to be treated, a film thickness measuring substrate 15and a deposition source 16 are disposed in the chamber 11. The filmthickness monitor 12 comprises a light source, a spectroscope and anoptical receiver. Light exiting from the spectroscope falls on the filmthickness measuring substrate 15, and the light reflected therefromenters the optical receiver whose output is sent to the film thicknesscontroller 13. In this way, the film thickness is monitored in real timesuch that the light absorption films and the dielectric films havingdesired thicknesses are formed on the substrate.

A vacuum gauge part 17, a vacuum gauge controller 18, a gas introductionunit 19 and an exhaust unit 20 are connected to the chamber 11. In thepresent embodiment, an APC method is adopted to maintain a constantdegree of vacuum in the chamber 11. Specifically, feedback is given viathe vacuum gauge part 17 and the vacuum gauge controller 18, and the gasintroduction unit 19 is controlled to regulate an amount of mixed gasintroduced into the chamber 11. However, the present invention is notlimited thereto, and may adopt a method of regulating the amount of gasintroduced into the chamber 11 to a constant amount by a needle valve.

FIG. 3 is a diagram representing film formation conditions when thevacuum deposition device shown in FIG. 2 is used to produce the NDfilter shown in FIG. 1. As shown, the substrate temperature is 100° C.Moreover, the ultimate vacuum of the chamber is set to 1×10⁻³ Pa.Herein, Ti is used as a raw material to form the light absorption films3, 5, and a deposition rate is set to 0.5 to 1.0 nm/sec. In the presentembodiment, air in which nitrogen and oxygen are mixed at 4:1 is used asa reactive gas to be introduced when Ti is deposited. However, thepresent invention is not limited thereto, and a mixed gas is generallyused which contains oxygen in a proportion of 50% or low. For example, amixed gas of O₂ and Ar can be used instead of a mixed gas of O₂ and N₂.It is to be noted that a deposition vacuum degree when a mixed gascontaining oxygen is introduced has been set to 3 to 4×10⁻³ Pa. However,the present invention is not limited thereto, and if it is maintained ata constant degree between 1×10⁻³ Pa to 1×10⁻² Pa, the light absorptionfilm can generally be formed which has satisfactory opticalcharacteristics and physical characteristics and which includes a metaland its saturated oxide as the main components and reduces theproportion of residual lower oxides. Subsequently, when the dielectricfilms 2, 4, 6 are formed, SiO₂ is used as the deposition source, and thedeposition rate is set to 0.5 to 1.0 nm/sec. When SiO₂ is formed, thereactive gas is not specifically introduced. In the present embodiment,the vacuum deposition is used to form the light absorption film.Instead, other PVD film formation methods that can form minute films canalso be utilized, such as an ion plating method, an ion assisted methodand a sputtering method.

FIG. 4 is a graph showing results of analyzing the composition of thelight absorption film formed by the reactive PVD on the conditions shownin FIG. 3. An X-ray photoelectron spectroscopic analyzer (XPS, ESCA) isused for this analysis. If soft X-rays having particular energy areapplied to a light absorption film surface in a high vacuum, electronsare released from a sample due to a photoelectric effect. These are ledto an analyzer, and separated in accordance with kinetic energy of theelectrons to be detected as a spectrum. FIG. 4 represents this spectrum.Photoelectrons are also released from deeper areas, but lose kineticenergy due to inelastic scattering before reaching the surface of thesample, so that they are not detected as a peak and will be a backgroundof the spectrum. The photoelectrons in an area several nm deep whichhave escaped from the sample surface without causing inelasticscattering are detected as a peak as shown in the graph and used for theanalysis. A horizontal axis of the spectrum in FIG. 4 indicates bondenergy of the electrons. The bond energy can be obtained by a differencein which the kinetic energy of the photoelectrons is subtracted from theenergy of the irradiated soft X-rays. Since inner shell electrons ofvarious atoms have intrinsic bond energy, the kind of element can bechecked from the bond energy of the detected electrons, and theproportion of element can be checked from signal strength. The spectrumin FIG. 4 is a result of detecting the bond energy of 2p inner shellelectrons of an atom. Further, if the chemical bonding states of thevarious elements are different, the bond energy slightly changes anddistinctively detected. This enables quantitative determination of themetal and its oxidation state. In the shown spectrum, the peak of metalTi is observed at 454.1 eV, the peak of its saturated oxide TiO₂ isobserved at 458.5 eV, the peak of lower oxide Ti₂O₃ is observed at 456.3eV, and the peak of another lower oxide TiO is observed at 455.2 eV. Itis to be noted that because the peaks of TiO and TiN emerge atsubstantially equal points, a peak of 455.2 eV seems to include TiN inaddition to TiO.

FIG. 5 is a table diagram representing the composition of the lightabsorption film calculated in accordance with the analysis shown in FIG.4. Looking at the proportion, metal Ti is 5%, TiO/TiN are 5%, Ti₂O₃ is10%, and TiO₂ is 80%. As shown in the table diagram of FIG. 5, thecomposition of the light absorption film formed on the conditions shownin FIG. 3 includes a Ti metal simple substance with saturated oxide TiO₂as the main components, and further includes the lower oxide mixed as aresidual component. It is to be noted that TiN is also presumed to bepresent because nitrogen is detected in the light absorption film. Theextinction coefficients of the light absorption films having such acomposition are about 0.5 to 1.0.

FIG. 6 shows results of analyzing element proportions in the lightabsorption film surface, which are also obtained by the XPS. Inaccordance with the shown table diagram, the element proportions of thelight absorption film are 53.8% of O, 27.5% of Ti, and 2.8% of N. Inaddition, 16.5% of other C is included, which seems to be a residualerror of organic matters such as an organic solvent and contaminationremaining on the surface of the light absorption film.

FIG. 7 is a graph showing the optical characteristics of the ND filterwhen the laminate structure shown in FIG. 1 is produced on the filmformation conditions shown in FIG. 3. The horizontal axis represents thewavelength in the visible area, and the vertical axis represents thelight amount (%) indicating the scale of reflectance and transmittance.As apparent from the graph, the present ND filter shows neutraltransmitting characteristics in the visible area, so that the ND filtercan be produced whose reflectance on the surface is kept low. It hasfurther been found out that when the present ND filter is subjected toan environmental test, it shows significantly satisfactory durability.In some cases, in order to stabilize unstable components such as thelower oxide included in the light absorption film, a heat treatment orthe like may be performed in an oxygen environment.

FIG. 8 is a schematic diagram in which the present ND filter is appliedto the light quantity diaphragming device for a camera. An ND filter 105is fixedly provided, for example, by an adhesive 106 or by thermalwelding, in a concave portion of illustrated one of aperture blades 100formed in pairs. The aperture blade 100 is configured to pivot around apivotal support pin 104 by a driving portion 103 to open/close anaperture 101.

FIG. 9 is a schematic sectional diagram showing a layer structure ofanother embodiment of the ND filter according to the present invention.In order to facilitate the understanding of the embodiment, likereference numerals are used to denote respective parts corresponding tothe previous embodiment shown in FIG. 1. As shown in the figure, Atransparent substrate 1 composed of PET has one face at an upper sidewhere dielectric films 2, 4 and 6 and light absorption films 3 and 5 arealternately superimposed. The transparent substrate 1 has an oppositeface at a rear side where an reflection prevention layer 7 is formed.This reflection prevention layer 7 is formed on the face different fromthe other face which is superimposed with the ND filter 0, in order tosuppress ghost or flea which would occur in an optical systemincorporating the ND filter 0. This reflection prevention layer 7 isformed of a single layer of a light absorption film or a dielectric filmso as to reduce reflection of light at the face opposite to the laminateface of the ND filter.

FIG. 10 is a schematic sectional diagram showing a layer structure of afurther embodiment of the ND filter according to the present invention.In order to facilitate the understanding of the embodiment, likereference numerals are used to denote respective parts corresponding tothe previous embodiment shown in FIG. 9. In this embodiment, anreflection prevention layer 7 is formed on the rear face of thetransparent substrate 1. Characterizingly, this reflection preventionlayer 7 is formed of a plurality of layers of a light absorption film 7a and a dielectric film 7 b, thereby obtaining more significant effectof suppressing reflection. The material of the dielectric film 7 b maybe selected not only from the materials used in the ND filter accordingto the present invention, but also selected from other materials (e.g.,SiO and MgF₂). Further, the material of the light absorption film 7 amay be selected not only from the material used in the ND filteraccording to the present invention, but also selected from othermaterials (e.g., Ta₂O₅, ZrO₂, TiO, TiO_(x)(1≦x≦2), Nb₂O₅, CeO₂ and ZnS).Further, The reflection prevention layer 7 may be formed of a mixture oftwo or more of these materials.

Alternatively, the reflection prevention layer 7 may be formed of one ormore of layers of thermosetting resin or optically setting resin whichis transparent in a visible light range. However, in case that ghost orflea hardly occurs in an optical system incorporating the ND filter,needless to say, it is not necessary to provide an reflection preventionlayer 7.

FIG. 11 is a schematic perspective exploded diagram showing anotherexample in which the ND filter according to the present invention isapplied to the light quantity diaphragming device for a camera. As shownin the figure, the light quantity diaphragming device for a camera isbasically composed of a base plate 201, a filter blade 202 and a coverplate 203. These parts are assembled by means of pins 207. The baseplate 201 has a circular aperture 204 which limits incident light. Thecover plate 203 has an aperture 205 of a diameter greater than theaperture 204 of the base plate 201. The base plate 201 and the coverplate 203 form therebetween a blade room where the filter blade 202 isarranged. The filter blade 202 is formed of the ND filter according tothe present invention, and has the same outer shape as a regularly useddiaphragm blade. This filter blade 202 is pivotally supported by a shaft(not shown) provided on the base plate 201, and structured toreciprocally move by a driving portion 206 between a position where thefilter blade 202 closes the apertures 204 and 205 and another positionwhere the filter blade 202 escapes from the apertures 204 and 205.

1. An ND filter in which light absorption films and dielectric films aresuperimposed on a transparent substrate, the ND filter characterized by:a composition of the light absorption films which includes 1 to 30 wt %of a pure component of a metal and 50 wt % or higher of a saturatedoxide component of the metal, and other residual components comprisingcompounds of the metal including lower oxides of the metal.
 2. The NDfilter according to claim 1, characterized in that a metal raw materialof the light absorption films is selected from Ti, Cr, Ni, NiCr, NiFeand NiTi.
 3. The ND filter according to claim 1, characterized in thatSiO₂ or Al₂O₃ is used for the dielectric films.
 4. The ND filteraccording to claim 1, characterized in that the light absorption filmsand the dielectric films are superimposed with predetermined thicknessand in a predetermined order to provide an reflection preventionfunction.
 5. The ND filter according to claim 1, characterized in that areflection prevention layer is provided on one face of the transparentsubstrate which is opposite to the other face of the transparentsubstrate where the light absorption films and the dielectric films aresuperimposed.
 6. The ND filter according to claim 5, characterized inthat the reflection prevention layer is formed of a single layer of alight absorption film or a dielectric film.
 7. The ND filter accordingto claim 5, characterized in that the reflection prevention layer isformed of a plurality of layers of a light absorption film and adielectric film.
 8. The ND filter according to claim 5, characterized inthat the reflection prevention layer is formed of one or more of layersof thermosetting resin or optically setting resin which is transparentin a visible light range.
 9. A light quantity diaphragming device usingthe ND filter according to claim 1.